1. Field of the Invention
The present invention relates to internetworking and in particular to packet multicasting.
2. Description of the Related Art
Conventional Internet Protocol (IP) multicast provides a multi-source group model in which receivers in a multicast group G receive packets from any source S on the Internet. Conventional IP multicasting is further described in Karanjit S. Siyan, Inside TCP/IP, 3d ed., New Writers Publishing, 1997, pages 426-429. IP multicasting is also described in U.S. Pat. No. 5,517,494 to Green entitled xe2x80x9cMethod and System of Multicast Routing for Groups with a Single Transmitterxe2x80x9d and M. Handley, xe2x80x9cInternet Multicast Today,xe2x80x9d The Internet Protocol Journal, December 1999, p. 2. All of these references are incorporated herein by reference in their entireties.
Recent work has recognized that major benefits in addressing, access control, and routing scalability are to be found from restricting multicast to a single-source multicast model. For example see, Hugh W. Holbrook and David R. Cheriton, xe2x80x9cIP Multicast Channels: EXPRESS Support for Large Scale Single-source Applicationsxe2x80x9d presented at SIGCOMM 1999, incorporated herein and by reference in its entirety. Most current applications for multicast are transmitted from a single-source or largely from a single-source out of a small group of sources. For example, video broadcasts, corporate file distribution, and stock quote distribution and dissemination applications are a few well-known, single-source applications. All essentially provide source data from one transmitting host.
One problem seen in the art is encountered when supporting multicast applications that are largely, but not entirely, single-source. Current state of the art approaches typically delegate the multicasting operation entirely to (slower) application level proxies, such as those provided by Akamai or FastForward. Other alternatives known in the art rely on complicated network and receiving host protocols. Such systems are described in U.S. Pat. Nos. 5,894,480 and 5,561,670, both to Hoffert et al. And both entitled xe2x80x9cMethod and Apparatus for Operating a Multicast System on an Unreliable Network.xe2x80x9d These patents describe variations on the well-known Internet Group Management Protocol (IGMP) and are incorporated by reference herein in their entireties.
All of these prior art methods share a common failing in that each is necessarily slowed by the overhead of the messaging protocols and OSI upper-layer application processing. This slow down also increases cost by constraining transmission speed, adding processing complexity, and increasing the probability of error.
What is needed is a simple, scaleable method of providing single-source multicasting from a small plurality of sources that appears to receiver groups as if it were originating from a unique, single-source host.
The present invention is a method for using network address translation (NAT) in switches and routers to define a virtual host as the source of a multicast channel within a single-source multicast model. This method translates packets from different (actual) multicast sources to the virtual host as part of the conventional packet forwarding process. Address-translated packets are thus forwarded through a single-source multicast channel and received by the subscribing host(s) as though the packets came from a single xe2x80x9cvirtualxe2x80x9d source.
This methodology can be used to map two or more sources simultaneously onto the same multicast channel. Such a mapping is useful, for example, to present multiple views of a sporting event video broadcast, provide advertisement insertion capability, or to support transparent fail-over to a backup video source in a critical multicast application.
Listening or receiving hosts in the multicast reception group simply subscribe to the single virtual host as the source of a multicast channel. It is not necessary for the recipient hosts to know the actual origin of the multicast data stream or its constituent parts; all that is required is that the recipient hosts subscribe to the proper multicast group channel and the virtual host address. The receiving group hosts are thus oblivious to the physical source of the multicast content. This application is analogous to a web site in which a single virtual host is actually implemented by multiple physical hosts, except the virtual host for the client machines in the web site case is the destination address to which they send, rather than the source address from which members of the multicast group receive.